There are a pile of variables. What kind of shape you are in. How hard you ride. How well you recover and so on. On many weekends, I do 50 miles on both days. I don't really notice any problems with recovering on the second day. Eating right and drinking right during and after the ride will help. That means whatever is "right" for you which again varies. I find after a hard ride, if I walk around, I recover faster. I think the extra blood flow to the leg muscles does it.